In the art there has hitherto been known a hydraulic motor in which a cylinder block is mounted in a housing so as to be axially rotatable. The cylinder block contains a cylinder bore in which a piston is slidably inserted providing a cylinder chamber such that with a leading end portion of the piston slidably driven along a swash plate an axial sliding movement of the piston may be effected. Fluid communication of the cylinder chamber alternate with a hydraulic supply and a reservoir allows the cylinder block to be axially rotated.
A known braking apparatus for a hydraulic motor of the type described is illustrated in FIG. 1. As illustrated, a cylinder block 1 and a housing 2 have rotatable side friction plates 3 and fixed side friction plates 4 attached respectively thereto and arranged together so that a former plate and a latter plate may be placed alternately. A piston 5, which is juxtaposed with a friction plate arrangement of these plates 3 and 4, is pushed by a spring 6 to bring the fixed side and rotation side friction plates 4 and 3 into a mutual pressure contact, thereby applying a braking to a movement of the cylinder block 1. Supplying a fluid under an elevated pressure into a pressure receiving chamber 7 for the piston 5 causes the piston 5 to be displaced against a resilient pressure by the spring 6 to separate the fixed side friction plates 4 and the rotation side friction plates 3 away from one another, thereby releasing the braking force applied to the cylinder block 1.
A further detail of the braking apparatus shown and described is schematically illustrated in FIG. 2 for a hydraulic motor 10. Disposed as juxtaposed with a rotating part 11 of the hydraulic motor 10 is a braking cylinder assembly 12 having a piston 13. The piston 13 is adapted to be displaced in a braking direction (here in the direction in which it is extended) as energized by a spring 14 and to be displaced in the opposite direction (here in the direction in which it is retracted) to release the braking when a piston pressure receiving cylinder chamber 15 is supplied with a pressure fluid.
A pressure fluid for supplying into the piston pressure receiving chamber 15 in the braking cylinder assembly 12 (a braking release pressure fluid) may well be an output pressure fluid delivered from a hydraulic pilot valve which is designed for a hydraulic motor, i.e. a valve to provide a pilot pressure fluid for switching an operating valve used to hydraulically drive the motor.
Typically, a hydraulic power shovel comprises a plurality of hydraulic actuators including a boom cylinder, an arm cylinder and a bucket cylinder, a plurality of operating hydraulic valves used to supply pressure fluid to these actuators, including a boom operating hydraulic valve, an arm operating hydraulic valve and a bucket operating hydraulic valve and a plurality of pilot valves for supplying pilot switching pressure fluid to these operating valves, including a boom associated pilot valve, an arm associated pilot valve and a bucket associated pilot valve. Each of these pilot valves and a hydraulic motor associated pilot valve mentioned in the preceding paragraph are coupled to and located at the discharge outlet of a single hydraulic pump.
The piston pressure receiving chamber 15 in the braking cylinder assembly 12 has a large pressure receiving area and also provides a long piston stroke in the braking release direction. Hence, displacing the piston 13 to the extent of its stroke end in order to release braking with the braking apparatus requires a plenty of pressure fluid to be supplied into the piston pressure receiving chamber 15 in the braking cylinder assembly 12.
In a compound operation in which the hydraulic motor and the arm are simultaneously operated to perform a turning operation and an arm control operation at the same time, supply of a plenty of pressure fluid from the hydraulic motor associated pilot valve into the piston pressure receiving chamber 15 in the braking cylinder assembly 12 extremely reduces the pressure of pilot pressure fluid, however. A delay may then be caused in the switching of the arm operating hydraulic valve by a failure of the piston 13 of the braking cylinder assembly 12 to be moved to the extent of its stroke end, deteriorating the operating performance of any of the other component associated hydraulic actuators in such a compound operation.
In the braking apparatus described, a means such as a switching valve may also be used to supply pressure fluid into the piston pressure receiving chamber 15 in the braking cylinder 12, or to allow pressure fluid to flow out of the piston pressure receiving chamber into a reservoir. It has then be experienced, however, that air tends to be entrapped in a circuit connecting the switching valve to the piston pressure receiving chamber of the braking cylinder assembly, a fluid passage in the switching valve and a circuit connecting the switching valve to the reservoir, assembled or while being assembled, and such air entrapment could seldom be expelled or extracted. The entrapment of air that remains lengthens the time which is elapsed actually for a breaking, i.e. the time from an instant at which the braking apparatus is acted on to commence releasing a breaking up to an instant when the fluid pressure in the piston pressure receiving chamber has been built up to a pre-established level to complete the braking release action.
It is accordingly an object of the present invention to provide a braking apparatus for a hydraulic motor, that can resolve the problem mentioned above.